This section provides background information related to the present disclosure which is not necessarily prior art.
Surfaces and materials with extreme repellency or attraction to liquids are of significant interest for a wide variety of military, commercial, and specialty applications. By way of non-limiting example, extreme repellency surfaces include those that are self-cleaning and non-fouling, including stain-free clothing and spill-resistant protective wear. Such extreme repellency surfaces may also be used for drag reduction, microfluidics, locomotion of micro-robots on aqueous and chemical environments, and for providing icephobicity. The primary measure of wetting of a liquid on a non-textured (or smooth) surface is the equilibrium contact angle θ (given by Young's relation). Non-textured surfaces that display contact angles θ greater than 90° with water are considered hydrophobic, while non-textured surfaces that display contact angles θ less than 90° with water are considered hydrophilic. Typically, surfaces with high surface energy tend to be hydrophilic, whereas those with low surface energy tend to be hydrophobic.
Relatively recently, a newer classification has emerged, known as a “superhydrophobic.” Superhydrophobic surfaces display contact angles θ greater than 150° along with a low contact angle hysteresis (the difference between the advancing and the receding contact angles) for water. Water droplets can easily roll-off from and bounce on such surfaces. Known superhydrophobic surfaces are textured (or rough), as the maximum water contact angle θ measured to date on a smooth surface is believed to be only about 130°. Superhydrophobic surfaces are pervasive in nature with various plant leaves, legs of the water strider, gecko's feet, troughs on the elytra of desert beetles, and insect wings displaying extreme water-repellency. Some synthetic or artificial engineered superhydrophobic surfaces have also been developed. These superhydrophobic surfaces tend to be quite difficult to reliably create, require complex processing and customized materials, and therefore have been quite expensive.
Surfaces that repel low surface tension liquids such as different oils are called superoleophobic. Most superoleophobic surfaces are also superhydrophobic, because surfaces that can repel low surface tension liquids (such as oils and alcohols) can much more easily repel water, which possesses a higher surface tension. However, there are a few superoleophobic surfaces that are at least partially wet by polar liquids such as water and alcohols. In view of such counter-intuitive surfaces, surfaces that can display both superhydrophobicity and superoleophobicity (e.g., as “omniphobic” surfaces) would be highly desirable. Similarly, an ability to create surfaces that exhibit other extreme wettabilities, such as surfaces that are both superhydrophilic (e.g., displaying contact angles θ of less than 5° for water) and superoleophobic or superhydrophobic and superoleophilic (e.g., displaying contact angles θ of less than 5° for oil) would also be highly desirable. There remains a need for improved, streamlined, cost-effective processes for forming surfaces having such extreme wettabilities, which can be used in a vast array of different technological fields and applications.